docs: def new(self, grp_id, branch_id):
          """
          :param grp_id: ordinal of the source group
          :param branch_name: name of the UCERF branch
          :param branch_id: string associated to the branch
          :returns: a new UCERFSource associated to the branch_id
          """
          new = copy.copy(self)
          new.orig = new
          new.src_group_id = grp_id
          new.source_id = branch_id
          new.idx_set = build_idx_set(branch_id, self.start_date)
          with h5py.File(self.source_file, "r") as hdf5:
              new.start = 0
              new.stop = len(hdf5[new.idx_set["mag"]])
          return new